The present invention relates to solder compositions, solder pastes and manufacturing methods thereof. More particularly, the present invention relates to a method of making solder powders and solder pastes substantially free of lead which provide for the relatively low operating temperatures associated with lead-based solders.
The electronic packaging and assembly industry is being pressured to convert to lead-free solders in response to growing concerns that the large number of electronic devices currently employing lead may create a public health risk over time. In response to the pressure, several lead-free solders have been proposed to replace the traditional tin-lead solders.
One such substitute is the family of tin-silver solders. However, this substitute is undesirable because of the increased operating temperature required to melt the solder compositions. Traditional tin-lead solder has a melting point of about 183xc2x0 C. Replacing the traditional tin-lead solder with a tin silver solder significantly increases the operating temperature to at least 215xc2x0 C. A concern about the use of these tin-silver solders is that such a higher operating temperature places significant strain on both the electronic components and the operating costs associated with these solders. This 30xc2x0 C. increase in temperature significantly increases costs of manufacturing electronic components because new materials for the circuit boards and electronic systems must be obtained to compensate for the increase in temperature they will be subjected to during processing.
Another potential solution is the use of tin-zinc solders. These tin-zinc solders are generally more desirable because the operating temperature is closer to the traditional tin-lead solders; however, these solders also have significant drawbacks. For example, zinc is a very reactive metal and oxidizes very quickly. This reactivity creates many problems when used in solder compositions. First, oxidized zinc creates wetting problems. Second, the zinc tends to react quickly with other components typically found in solder pastes, such as flux, thereby significantly reducing the shelf-life of the solders and solder pastes. Thus, in order to take advantage of the beneficial properties of tin-zinc solders, active steps must be taken to keep zinc from oxidizing and reacting.
Solutions to this problem often focus on the type of flux selected for use in conjunction with the solder powders. Other solutions include coating the zinc with another metal to slow the oxidation of the zinc. Typical coating procedures include hot-dipping, foiling and electroplating. One such coating solution is discussed in U.S. Pat. No. 6,186,390, which discloses a solder material and manufacturing method in which a solder material has a first layer (such as a sheet) composed of zinc and a second layer composed of tin. The two sheets are then rolled into a wire such that the tin layer surrounds the zinc layer. This coated wire creates a tin-zinc solder material in which the majority of the zinc is surrounded by tin. While this patent proposes a solution to the problem of zinc oxidation, the method of making these materials is extremely time consuming, likely very expensive and is not suitable for industrial processes for coating small particles already in powder form.
Other known coating methods such as hot dipping and electroplating, are also undesirable because of the nature of the materials selected and increased manufacturing costs. For example, hot dipping is generally not a viable solution for solder materials that have a lower melting temperature than the coating material itself.
Therefore, it would be desirable to provide a tin-zinc solder material and method for manufacturing thereof that takes advantage of the properties of tin-zinc solders, solves the problems of oxidation of zinc, and is easily and inexpensively manufactured.
The present invention provides for a substantially lead-free solder composition with improved wetting properties and extended shelf-life comprising a solder material and an immersion coating having a thickness between about 0.5 and 2 microns disposed about the solder material.
The solder composition is most preferably a metal alloy of tin and zinc coated with a coating material. The coating material is preferably selected from copper, silver, palladium, tin and gold. Additionally, the metal alloy contains a melting point reducing agent (xe2x80x9cXxe2x80x9d), to lower the melting temperature such that it is less than about 192xc2x0 C. The melting point reducing agent is an element preferably selected from indium, silver and bismuth. The resulting tin-zinc-X alloy is preferably provided in powder form with spheres approximately 30 microns in diameter. The coating is approximately 1 micron thick and is preferably tin.
The present invention also provides a method of making a coated solder composition comprising the steps of providing a solder material comprising tin and zinc and coating the solder material with about 0.5 to about 2 microns of a coating material by immersion coating or electroless plating. A method of making a solder paste is also provided by mixing the coated solder powder with an effective amount of flux.
These solder compositions and methods of manufacturing thereof provide significant advantages over the known solder compositions and the known methods of manufacturing these compositions.